1. Technical Field
The present invention relates to a block copolymer having excellent thermal decomposition resistance and oil resistance, exhibiting good compression set even at high temperatures, and being usable as a rubber, a thermoplastic resin, an impact modifier for thermoplastic resins, a compounding agent for a compound material having elasticity between those of resins and rubber, a paint, a binding agent, or an adhesive. More specifically, the present invention relates to a block copolymer containing a methacrylic polymer and an acrylic polymer and having excellent balance between physical properties, particularly thermal decomposition resistance and oil resistance, and good compression set at high temperatures.
The present invention further relates to a soft material for automobile containing a thermoplastic elastomer composition having low hardness, excellent adhesion, excellent heat resistance, excellent oil resistance, and excellent tensile properties (mechanical properties), and also having excellent flexibility, formability.
2. Background Art
In general, a thermoplastic elastomer has an alloy structure comprising a rubber component (soft segment) exhibiting entropy elasticity, and a restrictive component (hard segment) which is fluid at high temperatures and which prevents plastic deformation at room temperature and gives a reinforcement effect to the rubber component. For example, in a styrene elastomer, styrene blocks aggregate to function as a hard segment, whereas butadiene blocks or isoprene blocks form a matrix to function as a soft segment. An olefin elastomer has an alloy structure in which rubber such as EPDM is dispersed in a resin such as PP. In any of the elastomers, the hard segment is fluid at high temperatures, and thus permits thermoplastic processing such as injection molding. In addition to the styrene elastomer and the olefin elastomer, examples of thermoplastic elastomers include vinyl chloride elastomers, ester elastomers, amide elastomers, and urethane elastomers.
However, conventional styrene or olefin thermoplastic elastomer is insufficient in rubber elasticity, particularly compression set at high temperatures, as compared with crosslinked rubber. Therefore, thermoplastic elastomer exhibiting high rubber elasticity even at high temperatures is desired.
On the other hand, it is known that a (meth)acrylic block copolymer including a hard segment of methyl methacrylate or the like and a soft segment of butyl acrylate or the like is usable as a thermoplastic elastomer. As a block copolymer having a (meth)acrylic polymer block and an acrylic polymer block, the specification of Japanese Patent No. 2553134 discloses known examples such as a block copolymer (MMA-b-BA-b-MMA) of poly(methyl methacrylate)-b-poly(butyl acrylate)-b-poly(methyl methacrylate), and a block copolymer (MMA-b-2EHA-b-MMA) of poly(methyl methacrylate)-b-poly(2-ethylhexyl acrylate)-b-poly(methyl methacrylate). (Meth)acrylic block copolymers are characterized by excellent weather resistance, durability, heat resistance and oil resistance.
Acrylic block copolymers can be synthesized by various types of living polymerization. Examples of living polymerization include so-called group transfer polymerization in a silylketene acetal/Lewis acid system (Japanese Unexamined Patent Application Publication No. 62-292806), living polymerization using porphyrin-organoaluminum complex (S. Inoue and others, Macromolecules, Vol. 24, p. 824, 1991), and living polymerization using an organic rare earth metal complex as an initiator (Japanese Unexamined Patent Application Publication No. 6-93060). In particular, for polymerization using an organic rare earth metal complex, it has been reported that a (meth)acrylic polymer block is stereoregularly polymerized, and an acrylic polymer block is non-stereoregularly polymerized to form a block copolymer having excellent heat resistance and impact resistance or excellent elastomeric properties. Furthermore, in a method of atom transfer radical polymerization using a halogen-based initiator and a copper catalyst, an acrylic ester is polymerized, and then ester bonds are selectively decomposed to obtain a block polymer composed of polyacrylic acid-polyacrylic ester-polyacrylic acid (Japanese Unexamined Patent Application Publication No. 2001-234147). It is also introduced that the resultant block polymer can be used as an adhesive exhibiting a small change in physical properties even at high temperatures.
However, the hard segments of these (meth)acrylic block copolymers have glass transition temperatures of 150° C. or less, and thus have difficulty in exhibiting rubber characteristics at high temperatures which are required in the automobile field. Also, acrylic block polymers having hard segments of acrylic acid exhibit a breaking strength of less than 3 MPa (NITTO DENKO Technical Report (Nitto-Giho), Vol. 38, No. 2, November, 2000), and thus have the problem of failing to exhibit sufficient strength for use as elastomeric materials.
Furthermore, as an invention relating to the design of an adhesive material achieving a small change in physical properties even at high temperatures and having excellent balance between adhesive properties such as retention, an adhesive comprising a (meth)acrylic block copolymer having a hard segment of a polymethacrylic acid block has been reported (Japanese Unexamined Patent Application Publication No. 10-298248). Although this invention introduces a di-block polymer composed of polymethacrylic acid and polyacrylic ester in examples, di-block structures generally cannot express substantial compression set and breaking strength, which are important properties of elastomer, and thus the development of thermoplastic elastomer having excellent heat resistance is desired.
On the other hand, it is known that block copolymers can be used as compositions with thermoplastic resins. As such block copolymers, styrene-butadiene copolymers, styrene-isoprene copolymers, and hydrogenated polymers of these copolymers (called “styrene-ethylene-butylene copolymers”, and “styrene-ethylene-propylene copolymers”, respectively) are widely used. By using these block copolymers, compositions having excellent balance between impact resistance, rigidity, and forming fluidity can be obtained. However, the thermoplastic resins usable with the block copolymers are limited to low-polarity resins such as polystyrene resins, polyolefin resins, and polyphenylene ether resins.
Also, it has recently been known that a block copolymer containing a methacrylic polymer and an acrylic polymer functions as an excellent impact resistance modifier, and a combination with a thermoplastic resin produces a thermoplastic resin composition having excellent impact resistance. In this case, a high-polarity resin such as a polyvinyl chloride resin, a polymethyl methacrylate resin, a polycarbonate resin, a polyester resin, or a polyamide resin is effective as the thermoplastic resin.
The hard segment of a thermoplastic elastomer is fluid at high temperatures, and thus permits thermoplastic processing such as injection molding. However, when the thermal decomposition temperature of the thermoplastic elastomer is lower than the injection molding temperature, the thermoplastic elastomer may cause thermal deterioration. For example, the above-described high-polarity resin such as a polyvinyl chloride resin, a polymethyl methacrylate resin, a polycarbonate resin, a polyester resin, or a polyamide resin has both a high molding temperature and high heat resistance, and thus such a resin cannot be used for molding a mixture with the thermoplastic elastomer because the thermoplastic elastomer is thermally decomposed. In particular, most of the methacrylic polymers are decomposed to monomers by depolymerization at 170° C. to 250° C. (Polymer Handbook Third Edition, Wiley-Interscience, 1989). Therefore, when high-temperature thermal stability is required, an ester elastomer or an amide elastomer must be selected. However, such an elastomer may also be required to have improved physical properties including oil resistance, controlled balance of physical properties, and reduction of cost, and the development of a novel elastomer is greatly demanded.
A soft polyurethane (RIM urethane) material and a poly(vinyl chloride)-based (soft PVC-based) material have recently been used for molded products required a good touch, such as automobile parts. However, these materials have the problem of resources saving and environmental protection, i.e., the problem of recycling. The RIM urethane material is thermally curable, and the soft PVC contains a large amount of plasticizer and produces chlorine gas by heating. Thus, it is impossible or difficult to recycle these materials. Therefore, a resin alternative to the soft vinyl chloride resin and the polyurethane resin is demanded. A candidate of such an alternative material is a thermoplastic elastomer resin which is a rubber-like material, not requires a crosslinking process, and exhibits formability comparable to that of thermoplastic resins, and this thermoplastic elastomer resin has recently attracted attention in the field of automobile parts, household appliance parts, and construction materials.
Although conventional olefin thermoplastic elastomer and styrene thermoplastic elastomer are excellent in recycling property and mechanical properties, these elastomers have the problem of low adhesion to resins and metals and low oil resistance. Although the olefin thermoplastic elastomer is known to have proper flexibility and processability, the olefin thermoplastic elastomer does not exhibit sufficient weather resistance and oil resistance and cannot be directly used as an alternative. In particular, many processes for removing rustproof wax from an automobile often use a wax remover comprising a hot-water mixture containing kerosene and a surfactant. Thus, the use of the olefin thermoplastic elastomer as a material for exterior members has the problem of causing poor surface appearance during the removal of the wax.
Typical examples of hollow-molded products among the automobile parts include a boot and a hose. In particular, with respect to the boot, an accordion joint boot is mounted on a joint of an automobile or an industrial machine, for holding the sealed grease or preventing contamination with dust. Such a joint boot is conventionally made of rubber such as chloroprene, or a composition containing a monoolefin copolymer rubber and a polyolefin resin that are partially crosslinked with an organic peroxide used as a crosslinking assistant, as disclosed in Japanese Examined Patent Application Publication No. 53-21021.
From the viewpoint of recycling, thermoplastic polyester elastomers, which are non-crosslinking materials, have recently been used. Although the thermoplastic polyester elastomers are excellent in mechanical properties, heat resistance and oil resistance, the polyester elastomers have the problem of low flexibility due to high hardness, thereby significantly degrading mountability.
Therefore, in order to solve the problem, thermoplastic copolyester elastomer compositions comprising thermoplastic copolyester elastomers and rubber compositions are proposed in Japanese Unexamined Patent Application Publication Nos. 6-145477 and 7-97507. However, these thermoplastic copolyester compositions do not have flexibility and oil resistance sufficient for boots and hoses, and further investigation is required.
Also, Japanese Unexamined Patent Application Publication No. 2000-351889 discloses a thermoplastic elastomer composition comprising a thermoplastic copolyester elastomer and acrylic rubber. Although the oil resistance and boot-assembling performance are improved by mixing the acrylic rubber, molding fluidity decreases depending upon the content of the acrylic rubber. Therefore, molding is difficult, and the problem of flexibility remains unsolved, thereby causing the need for further improvement.
As described above, the thermoplastic polyester elastomer has excellent mechanical strength, but it has disadvantages of high hardness and poor oil resistance. Also, the acrylic rubber-compounded thermoplastic elastomer has improved oil resistance, but it still has disadvantages of poor formability and poor flexibility. Therefore, the application to automobile hollow-molded products is limited, and a material for automobile hollow-molded products having excellent oil resistance and excellent flexibility is required.